The present invention is generally directed to improvements in color television receivers. It is particularly directed to an improved VIR (vertical interval reference) correction system for use in such receivers.
Many television broadcast signals include a so-called VIR signal on line 19 of each television field. That VIR signal includes a VIR color burst and a VIR chrominance reference signal. Some television receivers now include a correction system which uses the VIR color burst and the VIR chrominance reference to automatically adjust the amplitude and phase of the color signal developed by the receiver.
Conventional VIR correction systems compare the amplitude of the demodulated VIR chroma reference to an amplitude reference signal generated in the receiver. Any deviation in the amplitude of the demodulated signal from the amplitude reference signal results in the generation of a feedback signal. The latter signal is used to adjust the gain of one or more amplifiers which amplify the VIR chroma reference prior to its demodulation. Because those same amplifiers process the normal, i.e., non-VIR color signal, the amplitude of the color signal is automatically controlled.
To control the phase of the color signal, the correction system compares the amplitude of the demodulated VIR color burst to another locally generated reference signal, and develops another feedback signal indicative of any difference between them. The latter signal is usually fed back to the receiver's tint control to adjust the phase of the 3.58 megahertz oscillator signal and thereby adjust the hue of the demodulated color signals.
One drawback of the correction system described above is its need for two reference signals, one for an amplitude reference and one for a phase reference. Both reference signals are normally at pre-set values which can be mis-adjusted. The added expense of providing the reference signals is, of course, also undesirable.
Another drawback resides in the fact that the receiver's usual color processing circuitry must be specially designed to interface with the VIR correction system. As stated above, the conventional correction system operates on demodulated color signals and is, therefore, located "downstream" of the receiver's color signal amplifiers, automatic color gain control circuitry, tint controls and the like which are included in the color processing circuitry. This arrangement presents several problems.
For example, if the viewer attempts to increase color level by adjustment of a customer control, the correction system tends to negate such adjustment by decreasing the gain of the color processing circuitry to hold color level constant. A similar effect occurs when the viewer attempts to adjust tint. To overcome this interaction, the color processing circuitry must be defeated or gated off during the nineteenth line of each field so that the customer controls do not affect the VIR signals. Thus, the color processing circuitry must be specially designed to include a provision for such gating.
Another undesirable effect occurs when the transmitted VIR signal is terminated. Assuming that the receiver includes the usual customer controls for adjusting color level and tint, and also includes pre-set (non-customer) color level and tint controls, the pre-set set controls may be "ON" when the VIR signal terminates. In that case, the pre-set controls operate to adjust color level and tint to presumably nominal settings. If, however, the pre-set controls are "OFF" when the VIR signal terminates, then the customer controls become effective. If those customer controls are mis-adjusted (as they are likely to be when the customer has been relying on VIR correction), then the color level and tint of the television image will be incorrect.
The present invention overcomes these problems to render VIR correction a more desirable feature in color television receivers.